There are several options presently available for use in concentrating solutions used in biomedical research and development. The solutions are aqueous in many instances, but may also frequently contain organic solvents that are miscible with water. Evaporation of organic solvents having low boiling points can often be performed using a standard "rotavapor" type device. Solvents having higher boiling points are typically removed by the use of distillation apparatuses. (e.g., a rotavapor of the RE-111 and RE-121 series, as are available from Buchi/Brinkmann through VWR Scientific).
Methods commonly used to remove water from solutions containing biopolymers typically involve lyophilization or freeze drying. For instance, such methods commonly employ a multipurpose freeze-drying apparatus such as that available from VWR Scientific as the Yamato Parkinson Freeze Dryer Model DC41-A, and the CD8 model multipurpose freeze dryer available from Heto Lab Equipment, Denmark. In such a method the sample is frozen in a tube or a flask, and a vacuum is applied. The removal of water is then performed from a solid state (e.g., ice) into a receiving flask, which is also cooled to collect the water vapor.
When an aqueous solution also contains an organic solvent that is miscible with water, it may not be possible to perform a lyophilization procedure, particularly if the mixture does not solidify upon cooling. In this case a number of other evaporation techniques can be used. Such a solution can be subjected to a vacuum, as described above, under conditions that prevent the sample from "bumping" (i.e., boiling in a manner that causes the solution to splash rapidly).
The solution can then be agitated or rotated to generate a centrifugal force. Agitation can be performed using, for instance, a Buchler vortex evaporator (e.g., model 09-548 available from Fisher Scientific, or Labconco RapidVap Models 16-317-1 and 16-317-3). Rotation can be performed using an instrument such as a New DNA SpeedVac instrument (e.g., Savant model DNA110, Jouan models RC 10.10 and RC 10.22, HETO models HS-1-60 and HS-1-110). Instruments of the former type provide a combination of gyrating motion, heat, and either vacuum or blowdown. Instruments of the latter type are generally referred to as centrifugal concentrators and are among the most common evaporators in laboratories.
With both types of evaporators, the evaporation process is generally facilitated by the use of a heating source, in fact, the vacuum chamber generally includes a thermostat-controlled heating device.
The vacuum needed to use such evaporators is typically provided by the use of a high-vacuum pump (e.g., HETO RZ 2 type) that is capable of generating pressure down to at least 1 mm Hg. Such pumps are often oil-type pumps that must be protected from solvents and water by the use of cooling traps (e.g., HETO CT60e type), in order to condense the evaporated solvent before it reaches the oil. Alternatively, such a pump can be a diaphragm-type that does not require the use of oil, but which typically does not generate as much vacuum (for instance, down to 10-20 mm Hg), thereby requiring longer evaporation times.
A simple evaporation process can be achieved by the use of a heating block such as the "Reacti-Therm" dry block available from Pierce as product #18800/18801. In such a process a tube containing a sample is inserted into a block that can be heated to a desired temperature.
A more efficient evaporation is achieved in the heated block if a gas is streaming through to carry the gaseous solvent with it. These types of evaporators are commonly called blow-down evaporators (Pierce, Reacti-Vap evaporator #18780). Examples of such evaporators include hot-blocks from Pierce and hot-block/blow-down combinations as described above. A type of blow-down unit has recently become available from Zymark ("TurboVap LV"). This unit involves the use of air or gas fed into a tube that is kept in a water-bath, in order to generate what is described as a gas vortex shearing technique.
The evaporation units presently available, however, generally continue to encounter significant drawbacks. Generally, the evaporation units that are the least expensive and easiest to use are the same units that require the longest evaporation times, or have other associated drawbacks. In contrast, the more expensive and technically complicated units are often unnecessary for simple procedures and cost-prohibitive for many labs.
What is clearly needed is an evaporation unit that provides an optimal combination of cost, ease of use, and efficiency for evaporating aqueous solvents commonly encountered in biomedical research.